DNA-binding proteins are proteins that comprise any of many DNA-binding domains and thus have a specific or general affinity to DNA. DNA-binding proteins include for example transcription factors that modulate the process of transcription, nucleases that cleave DNA molecules, and histones that are involved in DNA packaging in the cell nucleus.
The AT-hook motif is a short DNA binding protein motif that was first described in the high mobility group non-histone chromosomal proteins, HMG-I/Y (Reeves and Nissen (1990) J Biol Chem 265: 8573-8582). The AT-hook is known to interact with the minor groove of AT-rich nucleic acid sequences (Huth et al. (1997) Nat Struc Biol 4: 657-665). AT-hook motifs have been identified in a wide variety of DNA binding proteins from animals, plants and microorganisms. Unlike several well-characterized DNA binding motifs, the AT-hook motif is short, up to 13 amino acid residues, and has a typical tripeptide sequence with a glycine-arginine-proline (Gly-Arg-Pro or GRP) at its center.
In Arabidopsis thaliana, approximately 30 polypeptides, comprising at least one AT-hook motif, further comprise a plant and prokaryotes conserved (PPC) domain, which is described as DUF296 (domain of unknown function 296) in the InterPro domain database of the European Bioinformatics Institute (EBI) (Fujimoto et al. (2004) Plant Molec Biol 56: 225-239). One of these proteins was found to be localized in the nucleoplasm, and therefore named AT-hook motif nuclear localized protein 1 (AHL1; Fujimoto et al., supra). The paralogous polypeptides were similarly named, i.e. AHL, and numbered consecutively.
In U.S. Pat. No. 7,193,129, and in US patent application 2005/0097638, an Arabidopsis thaliana AHL polypeptide, AHL19 (according to Fujimoto et al., supra) (identified as G2153) was transformed into Arabidopsis, and expressed using the 35S CaMV promoter. Transgenic plants showed modified traits, such as increased salt stress resistance, increased osmotic stress resistance, increased drought resistance, increased tolerance to freezing and increased plant response to sugars. In US patent application 2005/0097638, overexpression (under the control of a 35S CaMV promoter) of AHL19 polypeptide, as well as of several paralogous AHL polypeptides, significantly delayed flowering in the transgenic plants compared to control plants, thereby increasing yield.